Rectifiers of various designs may be used for feeding direct current systems out of three-phase current systems, of motor vehicle electrical systems, for example, using three-phase current generators. Bridge rectifiers having a six-, eight-, or ten-pulse design are used in motor vehicle electrical systems, corresponding to the three-, four-, or five-phase current generators which are usually installed. However, the present invention is also suitable for bridge rectifiers having other numbers of phases.
When reference is made below to a generator for the purpose of simplicity, this may also be an electric machine which is operable in a generator mode and a motor mode, for example a starter generator.
A so-called load dump is a critical operating condition in bridge rectifiers. A load dump occurs when, for a highly excited generator and a correspondingly high delivered current, the load on the generator or the bridge rectifier connected thereto suddenly decreases, for example by disconnecting consumers. As a result of the energy which continues to be delivered by the generator, high voltages occur for approximately 300 ms to 500 ms. If it is not possible to intercept or completely intercept these high voltages by capacitively acting elements in the direct voltage network (for example, the motor vehicle battery) or in the rectifier, components in the motor vehicle electrical system may be damaged due to overvoltages.
In conventional (passive) bridge rectifiers, a certain amount of protection is provided by the rectifier itself, namely, with the aid of the rectifier Zener diodes, installed there in the classical case, in which the overvoltage may be arrested and the excess energy may be converted into heat.
However, as described in German Patent Application No. DE 10 2009 046 955 A1, for example, the use of active or controlled bridge rectifiers is desirable in motor vehicles. This is the case, among other reasons, due to the fact that active bridge rectifiers, in contrast to passive or uncontrolled bridge rectifiers, have lower power losses during normal operation. However, presently available controllable or active switching elements for such active bridge rectifiers, for example MOSFETs, have no integrated arresting function with sufficient robustness, as is the case for conventional rectifier Zener diodes, and therefore are not able to intercept the overvoltage. For this reason, additional protective strategies are necessary in active bridge rectifiers.
During a load dump, the generator phases for example may be short-circuited by temporarily conductively connecting some or all switching elements of the upper or lower rectifier branch of a corresponding rectifier, for example as also described in German Patent Application No. DE 198 35 316 A1 and German Patent Application No. DE 10 2009 046 955 A1. This takes place in particular on the basis of an evaluation of the output voltage present at the direct voltage terminals of the active bridge rectifier. If the output voltage exceeds a predefined upper threshold value, a corresponding short circuit is initiated and the output voltage drops. If the output voltage subsequently falls below a predefined lower threshold value, the short circuit is eliminated and the output voltage rises again. Therefore, this is typical hysteresis behavior. Thus, during a load dump the output voltage oscillates between the upper and the lower threshold value.
Problems may thus arise here, since the mentioned switching operations, i.e., repeating phase short circuits, result in heavy stress on the involved switching elements. This may damage the switching elements when they are not appropriately dimensioned.
Therefore, there is a need for improved protective strategies for active bridge rectifiers in the event of a load dump.